The present invention relates to a technique of driving a flat type image display device used in a display for the like of a television receiver or a computer.
As one of the prior arts is known an image display device described by U.S. Pat. No. 4,227,117 assigned to the assignee of the present application. FIG. 12 is a view showing the construction of a flat type cathode-ray tube which has an internal structure slightly different from that of the display device of the U.S. Pat. No. 4,227,117 but displays an image in accordance with substantially the same principle as the display device of U.S. Pat. No. 4,227,117. The display device shown in FIG. 12A or FIG. 12B includes line-like thermionic cathodes 1 (hereinafter referred to as line cathodes) as electron beam emitting sources, a back face electrode 2 which is disposed opposing the line cathodes 1 and on a side reverse to an image display plane, and a plate-like electron beam extracting electrode 3, an electron beam modulating electrode 4, a vertical focusing electrode 5, a horizontal focusing electrode 6, horizontal deflection electrodes 7, 7', vertical deflection electrodes 8, 8' and a phosphor-coated screen 9 which are successively disposed opposing the line cathodes 1 and on the same side as the image display plane. Here, FIG. 12B is a structural drawing showing a practical structure of conventional flat type cathode-ray tube corresponding to the cathode-ray tube shown in FIG. 12A. These components are accommodated in a flat vacuum glass vessel (not shown).
Each line cathode 1 functioning as the beam source is stretched in a horizontal direction and a plurality of such line cathodes (L in the explanation and four in the illustration in FIG. 12A or FIG. 12B) are disposed at proper intervals along a vertical direction. The line cathode 1 has a structure in which an oxide cathode material is applied on the surface of a tungsten filament of, for example, 20 to 30 .mu.m.phi..
The back face electrode 2 made of a conductive plate which may be planar, has a function of suppressing the generation of an electron beam or pushing the generated electron beam toward the display plane side.
The electron beam extracting electrode 3 is a plate-like electrode having M beam transmissive apertures which are provided at each of locations opposite to the line cathodes 1-1 to 1-L and at proper intervals in the horizontal direction. A part of an electron beam extracted from the heated line cathode 1 toward the display plane side by the electron beam extracting electrode 3 is passed through the apertures of the electron beam extracting electrode 3. When passed through the apertures, the beam is divided into M beams in the horizontal direction.
The electron beam modulating electrode 4 provided next to the electron beam extracting electrode 3 is divided into M segments in the horizontal direction so as to permit independent and simultaneous control of the quantities of transmission of the M divisional electron beams from the beam transmissive apertures of the electron beam extracting electrode 3. Only four segments of the electron beam modulating electrode 4 are shown.
The vertical focusing electrode 5 or the horizontal focusing electrode 6 has slits elongated in the vertical direction or the horizontal direction or apertures elongated in the vertical direction or the horizontal direction and serves to focus each beam in the vertical direction or the horizontal direction.
The horizontal deflection electrode includes M pairs of electrodes 7 and 7' with each of the divisional electron beams being sandwiched between the one pair of electrodes 7 and 7' on opposite sides in the horizontal direction. The beam is deflected in the horizontal direction by virtue of a potential difference applied between the paired electrodes 7 and 7'. Since the electrodes 7 in the M pairs and the electrodes 7' in the M pairs are connected by respective common buses or frames 12, the deflection is made for M beams for each line all at once.
The vertical deflection electrode includes L pairs of electrodes 8 and 8' with all of the beams for one line being sandwiched between one pair of electrodes 8 and 8' on opposite sides in the vertical direction. Each beam is deflected in the vertical direction by virtue of a potential difference applied between the paired electrodes 8 and 8'. The electrodes 8 in the L pairs and the electrodes 8' in the L pairs are connected by respective common buses or frames 12 so as to drive the beams such that the directions of vertical deflection of the beams corresponding to adjacent line cathodes 1 are reversed to each other.
Electron beams subjected to the focusing, modulation and deflection mentioned above are accelerated by a high voltage applied to the screen 9 so that the electron beams impinge upon phosphors on the screen 9 to excite the phosphors into luminescence. The screen 9 is formed by applying three-color (R, G and B) phosphors in stripe shapes with blacks therebetween on a glass plate and depositing a metal back layer on the phosphor stripes. The phosphor stripe is formed, for example, so that one pair of R, G and B (or one triplet) correspond to each of the beam transmissive apertures of the electron beam modulating electrode 4. Each of image display sections 10 shown by broken lines in FIG. 12 represents a region where an image is displayed by one beam which is passed through the modulating electrode 4 and is deflected in the vertical and horizontal directions. The plurality of image display sections 10 are connected on the screen 9 to display one image as a whole.
Next, a method of driving the conventional display device will be explained by use of FIG. 13 which shows a block diagram of the basic driving circuit and FIG. 14 which shows the waveforms of driving signals for the respective electrodes.
Reference signals for driving are a vertical synchronizing signal V.D, a horizontal synchronizing signal H.D which are separated from a television video signal 21 at a sync separator circuit 22 and a clock signal generated at a system clock generating circuit 32. The explanation will be made supposing a video signal in an NTSC system. Now, assume that the number of the line cathodes 1 is L. Then, in an effective vertical scanning period of a vertical scanning period IV excepting a vertical blanking interval (or a period of 240H corresponding to 240 horizontal scanning periods), L pulses k.sub.1 to k.sub.L having different phases and each having a low potential during only a period of time corresponding to the width of (240/L)H are generated and are successively applied to the line cathodes 1-1 to 1-L. The cathode driving pulses are generated in such a manner that a pulse having a pulse width of (240/L)H is sequentially shifted in a line cathode driving circuit 24 by virtue of trigger pulses p.sub.1 to p.sub.L each of which is generated by a vertical driving pulse generating circuit 23 each time it counts 240/L horizontal synchronizing signals H.D.. The back face electrode 2 is applied with a DC potential V.sub.2 which is slightly lower than the low potential level of the pulse applied to the line cathode, and the beam extracting electrode 3 is applied with a DC potential V.sub.3 which is sufficiently higher than the low potential level of the pulse applied to the line cathode. V.sub.2 and V.sub.3 are supplied from a power source circuit 20. During a period of time when the potential of the line cathode 1 is high, the cathode is heated but no electrons are extracted from the cathode. Only in periods of time when the potentials of the line cathodes 1-1 to 1-L are made low by the pulses k.sub.1 to k.sub.L, electron beams are successively extracted from the line cathodes 1-1 to 1-L.
The extracted electron beam is modulated in accordance with a video signal voltage including image information. In order to display a color image, it is necessary to excite three R, G and B phosphors into luminescence for R, G and B video signals, respectively. In the illustrated example, there is employed a method in which R, G and B video signals are successively applied to the modulating electrodes 4-1 to 4-M on the time-sequential basis in synchronism with the horizontal deflection. A video signal 21 is demodulated by a color demodulation circuit 34 into R, G and B signals which in turn are digitized by A/D converters 25-1 to 25-M at each timing triggered by pulses S.sub.1 to S.sub.M generated at a sampling pulse generating circuit 33 and are then held in video memories 26-1 to 26-M for a period of time 1H. The held data are sent to modulating circuits 27-1 to 27-M in a period of time for change-over of 1H in accordance with a read-out pulse f. In the modulating circuits, the digital data are converted into analog signals having pulse widths proportional to the values of data or analog signals having pulse amplitudes proportional to the values of data. The analog signals are applied to the beam modulating electrodes 4-1 to 4-M in the form of a serial signal of R, G and B by switching pulses S.sub.R, S.sub.G and S.sub.B which are generated at a switching pulse generating circuit 28 synchronizing horizontal driving pulses r, g and b which are generated at a horizontal driving pulse generating circuit 29. An example of the modulation signal is shown in FIG. 14, as w. The timings of application of the R, G and B pulses are matched to periods of time when the beam is resting on R, G and B phosphors at three steps synchronizing the horizontal driving pulses r, g and b in one horizontal deflection period 1H. Since (M) video signals for one line in the horizontal direction can be simultaneously applied to the modulating electrode 4, there is provided a line-sequential or line-by-line display system in which an image for one line can be displayed at a time.
The modulated electron beam is focused in the vertical and horizontal directions by DC potentials V.sub.5 and V.sub.6, which are generated at the power circuit 20, applied to the vertical focusing electrode 5 and the horizontal focusing electrode 6 and is thereafter electrostatically deflected by the horizontal deflection electrodes 7 and 7'. The deflection is effected by stepped deflection waveforms h and h' shown in FIG. 14 which are generated at a horizontal deflection driving circuit 30. Provided that the deflection width is selected to be equal to one triplet of R, G and B, the deflection waveform h or h' synchnous with the horizontal synchronizing signal H.D takes a stepped waveform in which the voltage is step-wise raised or lowered at every H/3 period synchronizing the horizontal driving pulses r, g and b. Accordingly, the electron beam rests on the R, G and B phosphors for H/3 periods, respectively.
On the other hand, the deflection in the vertical direction is effected by stepped deflection waveforms v and v' which are generated at a vertical deflection driving circuit 31. Since a period of time when the beam is extracted from each cathode is (240/L)H, each beam is deflected with (240/L) steps (in the shown example, 240/80=3 steps on the assumption that L=80) in the vertical direction or the deflection over the whole of the screen is made with 240 steps in total in one vertical scanning period (or one field) to depict 240 rasters. In the next field, the voltage value of the vertical deflection waveform is shifted so that beams land between the rasters depicted in the preceding field. Namely, an interlace scanning is performed.
The horizontal deflection and the vertical deflection are made in the above manner so that one image display section 10 is formed by 3 (in the vertical direction).times.3 (in the horizontal direction) spots excited into luminescence by one electron beam accelerated by a high voltage V.sub.9 applied to the screen 9, and such image display sections 10 are regularly arranged on the screen 9 to provide one image.
In the above flat type cathode-ray tube described as the prior art or another display device with a deflection of a plurality of electron beams, the uniformity of image quality is deteriorated unless the landing state of the electron beam on the screen 9 is uniform at any point.
Because the non-uniformity of beam landing positions and luminous spot shapes in the vertical direction appears as stepped brightness differences at the boundary portions between adjacent image display sections and the non-uniformity of landing positions and luminous spot shapes in the horizontal direction appears as stepped brightness differences or color differences at the boundary portions. The non-uniformity of beam landing positions is caused from the precision of work and the precision of assemblage of electrodes which contribute to vertical deflection or horizontal deflection in the flat type cathode-ray tube. However, as an area where an image is to be displayed is enlarged, it becomes difficult in view of technique and cost to enhance each of the precision of work and the precision of assemblage up to a level at which the image is not affected. Therefore, attempts to control the beam landing by use of electrical means have been proposed by, for example, U.S. Pat. No. 4,451,852. However, since it is not possible to drive deflection electrodes separately for individual beams, it was not possible to eliminate localized non-uniformity of landing.
The non-uniformity is shape of luminous spots is caused from a change in focusing characteristic of a beam depending on the degree of deflection of the beam which change is produced since the screen plane is planar of flat. This may be prevented by making the deflection angle as small as possible. For accomplishment of that purpose in a large-area display device may be considered, for example, a measure in which the deflection angle in the vertical direction is reduced by increasing the number of line cathodes and the deflection angle in the horizontal direction is reduced by increasing the number of electron beams into which an electron beam from one line cathode is to be divided. However, this measure is not proper since there results in an increase of a power consumption required for heating and an increase in cost or the precision of work electrodes in cost or the precision of work electrodes must be further enhanced. Also, there was not a method of coping with localized non-uniformity in shape of spots.